1. Field of the Invention
The present invention relates to a technology for manufacturing an optical fiber.
2. Description of the Related Art
An optical fiber is generally manufactured from an optical-fiber preform set in a draw furnace of a draw tower. Specifically, the distal end of the optical-fiber preform is heated to be melted to draw the optical fiber, and a single-layer or a multilayer ultraviolet (UV) curable resin and the like is applied on the optical fiber, fabricating a coated optical fiber.
In the above manufacturing process, it is extremely difficult to form perfectly circular core and cladding in a concentric manner in the longitudinal direction of the optical fiber; and therefore, a typical cross section of the core or the cladding is elliptical or distorted, or the core and the cladding are formed in an eccentric manner. Such imperfections cause difference between propagation velocities of lights propagating in two different polarization modes through the optical fiber, causing large polarization mode dispersion (PMD). If an optical fiber with large PMD is used as a transmission line for an optical-fiber communication, various problems may be induced.
Japanese Patent Application Laid-open No. H6-171970 and Japanese Patent Application Laid-open No. 2003-146689 disclose conventional technologies for reducing the PMD in an optical fiber. Specifically, the PMD is reduced by periodically applying the same amount of glass spin in the clockwise direction and the counterclockwise direction to a coated optical fiber.
However, when the glass spin is applied to an optical fiber in the conventional technologies, a residual torsional stress in either of the directions may be present on the optical fiber. Hereafter it is called “mechanical twist” in this specification. If the mechanical twist is present, the PMD in the optical fiber may increase more than that without the glass spin. Furthermore, the mechanical twist is a torsional stress with an intrinsic stress acting against the residual stress. Therefore, if an optical fiber having the mechanical twist is used for manufacturing an optical-fiber ribbon, the optical fiber may be curled, causing degradation of operation efficiency.
A conventional technology for reducing the mechanical twist is disclosed in Japanese Patent Application Laid-open No. 2006-315913. Specifically, after applying a glass spin to a coated optical fiber, a glass spin in a direction opposite to a direction of the mechanical twist is further applied to the optical fiber to reduce the mechanical twist.
Recently, a transmission speed in an optical fiber communication is increasing, requiring more reduction of the PMD in an optical fiber of a transmission line.
However, it is difficult, in the conventional technology, to sufficiently reduce the mechanical twist in the longitudinal direction of an optical fiber. Consequently, the PMD is not sufficiently reduced with the conventional technology.